SOFTWARE ENGINEERING Chapter 7 –Detail Design

Topics covered •Objectoriented design using the UML •Design patterns •Open source development Design and implementation •Software design and implementation is the stage in the software engineering process at which an executable software system is developed. •Software design and implementation activities are invariably interleaved. Jul 2013Chapter 7. Software Detail Design3 invariably interleaved. •Software design is a creative activity in which you identify software components and their relationships, based on a customer’s requirements. •Implementation is the process

Chapter 7 – Detail Design

Design and implementation

software engineering process at which an executable

software system is developed

Build or buy

off-the-shelf systems (COTS) that can be adapted and

tailored to the users’ requirements

• For example, if you want to implement a medical records system, you can buy a package that is already used in hospitals It can be cheaper and faster to use this approach rather than developing a system in a conventional programming language.

process becomes concerned with how to use the

configuration features of that system to deliver the system requirements

An object-oriented design process

developing a number of different system models

maintenance of these models and, for small systems, this may not be cost-effective

may not be cost-effective

design models are an important communication

mechanism

Process stages

processes that depend on the organization using the process

• Define the context and modes of use of the system;

• Define the context and modes of use of the system;

• Design the system architecture;

• Identify the principal system objects;

• Develop design models;

• Specify object interfaces.

weather station

System context and interactions

that is being designed and its external environment is

essential for deciding how to provide the required system functionality and how to structure the system to

communicate with its environment

communicate with its environment

boundaries of the system Setting the system boundaries helps you decide what features are implemented in the system being designed and what features are in other

associated systems

Context and interaction models

demonstrates the other systems in the environment of the system being developed

the system interacts with its environment as it is used

the system interacts with its environment as it is used

System context for the weather station

Weather station use cases

Use case description—Report weather

System Weather station

Use case Report weather

Actors Weather information system, Weather station

Description The weather station sends a summary of the weather data that has

been collected from the instruments in the collection period to the weather information system The data sent are the maximum, minimum, and average ground and air temperatures; the maximum, minimum, and and average ground and air temperatures; the maximum, minimum, and average air pressures; the maximum, minimum, and average wind

speeds; the total rainfall; and the wind direction as sampled at minute intervals.

five-Stimulus The weather information system establishes a satellite communication

link with the weather station and requests transmission of the data.

Response The summarized data is sent to the weather information system.

Comments Weather stations are usually asked to report once per hour but this

frequency may differ from one station to another and may be modified in the future.

Architectural design

environment have been understood, you use this

information for designing the system architecture

system and their interactions, and then may organize the

system and their interactions, and then may organize the components using an architectural pattern such as a

layered or client-server model

subsystems that communicate by broadcasting messages

on a common infrastructure

station

Architecture of data collection system

Object class identification

• Identifying object classes is toften a difficult part of object oriented design

relies on the skill, experience and domain knowledge of system designers

system designers

unlikely to get it right first time

Approaches to identification

description of the system (used in Hood OOD method)

domain

what participates in what behaviour

and methods in each scenario are identified

Weather station description

• A weather station is a package of software controlled

instruments which collects data, performs some data

processing and transmits this data for further processing The instruments include air and ground thermometers, an anemometer, a wind vane, a barometer and a rain gauge

anemometer, a wind vane, a barometer and a rain gauge Data is collected periodically

the weather station processes and summarises the

collected data The summarised data is transmitted to the mapping computer when a request is received

Weather station object classes

may be based on the tangible hardware and data in the system:

• Ground thermometer, Anemometer, Barometer

• Application domain objects that are ‘hardware’ objects related to the

• Application domain objects that are ‘hardware’ objects related to the

instruments in the system.

Weather station object classes

Design models

relationships between these entities

terms of object classes and relationships

between objects

Examples of design models

into coherent subsystems

interactions

change their state in response to events

models, generalisation models, etc

Subsystem models

groups of objects

encapsulation construct This is a logical model The

actual organisation of objects in the system may be

actual organisation of objects in the system may be

different

Sequence models

interactions that take place

• Objects are arranged horizontally across the top;

• Time is represented vertically so models are read top to bottom;

• Interactions are represented by labelled arrows, Different styles of

• Interactions are represented by labelled arrows, Different styles of arrow represent different types of interaction;

• A thin rectangle in an object lifeline represents the time when the object is the controlling object in the system.

collection

Character Use Case

1.1 create; display

:Engagement Display :Engagement

1.2 create

:Player Character

:Encounter

Game

freddie:

Foreign Character

2.1 execute

3.1 Display result

3.2 create 2.2 change quality values

1.2 create

Character Use Case

freddie:

Foreign Character

1.2 display()

:Encounter Cast

1.1 displayForeignChar()

2 execute()

2.1 setPlayerQuality()

2.3 setForeignQuality() 1.3 new Engagement()

3.2 displayResult()

:Player’s main character

Sequence diagram: srs vs sdd

State diagrams

different service requests and the state transitions

triggered by these requests

or an object’s run-time behavior

or an object’s run-time behavior

objects in the system Many of the objects in a system are relatively simple and a state model adds unnecessary

detail to the design

Weather station state diagram

Interface specification

and other components can be designed in parallel

representation but should hide this in the object itself

on the methods provided

but Java may also be used

Weather station interfaces

Design patterns

about a problem and its solution

of its solution

settings

characteristics such as inheritance and polymorphism

• Not a concrete design but a template for a design solution that can

• Not a concrete design but a template for a design solution that can

be instantiated in different ways.

• The results and trade-offs of applying the pattern.

The Observer pattern

The Observer pattern (1)

Pattern

name

Observer

Description Separates the display of the state of an object from the object itself and

allows alternative displays to be provided When the object state changes, all displays are automatically notified and updated to reflect the change.

Problem In many situations, you have to provide multiple displays of state

description information, such as a graphical display and a tabular display Not all of

these may be known when the information is specified All alternative presentations should support interaction and, when the state is changed, all displays must be updated.

This pattern may be used in all situations where more than one display format for state information is required and where it is not

necessary for the object that maintains the state information to know about the specific display formats used.

The Observer pattern (2)

Pattern name Observer

Solution

description

This involves two abstract objects, Subject and Observer, and two concrete objects, ConcreteSubject and ConcreteObject, which inherit the attributes of the related abstract objects The abstract objects include general operations that are applicable in all situations The state to be displayed is maintained in

ConcreteSubject, which inherits operations from Subject allowing it to add and remove Observers (each observer corresponds to a display) and to issue a notification when the state has changed.

notification when the state has changed.

The ConcreteObserver maintains a copy of the state of ConcreteSubject and implements the Update() interface of Observer that allows these copies to be kept

in step The ConcreteObserver automatically displays the state and reflects changes whenever the state is updated.

Consequences The subject only knows the abstract Observer and does not know details of the

concrete class Therefore there is minimal coupling between these objects

Because of this lack of knowledge, optimizations that enhance display performance are impractical Changes to the subject may cause a set of linked updates to observers to be generated, some of which may not be necessary.

pattern

A UML model of the Observer pattern

Design problems

any design problem you are facing may have an

associated pattern that can be applied

• Tell several objects that the state of some other object has changed (Observer pattern).

• Tidy up the interfaces to a number of related objects that have often been developed incrementally (Façade pattern).

• Provide a standard way of accessing the elements in a collection, irrespective of how that collection is implemented (Iterator pattern).

• Allow for the possibility of extending the functionality of an existing class at run-time (Decorator pattern).

developed from scratch, by writing all code in a high-level programming language

• The only significant reuse or software was the reuse of functions and objects in programming language libraries

became increasingly unviable, especially for commercial and Internet-based systems

existing software emerged and is now generally used for business and scientific software

Reuse levels

• At this level, you don’t reuse software directly but use knowledge of successful abstractions in the design of your software

• At this level, you directly reuse objects from a library rather than

• At this level, you directly reuse objects from a library rather than writing the code yourself

• Components are collections of objects and object classes that you reuse in application systems

• At this level, you reuse entire application systems

Reuse costs

reuse and assessing whether or not it meets your needs

software For large off-the-shelf systems, these costs can

be very high

be very high

software components or systems to reflect the

requirements of the system that you are developing

each other (if you are using software from different

sources) and with the new code that you have developed

Development platform tools

that allows you to create, edit and compile code

set of tests on a new version of a program

different development projects

an integrated development environment (IDE)

aspects of software development, within some common framework and user interface

framework and user interface

programming language such as Java The language IDE may be developed specially, or may be an instantiation of

a general-purpose IDE, with specific language-support tools

Component/system deployment factors

architecture, or relies on some other software system, it must obviously be deployed on a platform that provides the required hardware and software support

deployed on more than one platform This means that, in the event of platform failure, an alternative implementation

of the component is available

components, it usually makes sense to deploy them on

the same platform or on platforms that are physically

close to one other This reduces the delay between the

time a message is sent by one component and received

by another

Midterm Exam Revisited (Again)

Xét một hệ thống phần mềm hỗ trợ hoạt động của một chuỗi các cửa hàng bán lẻ thuộc tập đoàn Z chuyên cung cấp các mặt hàng phục vụ khách tại các nhà ga xe lửa như nhật báo, kẹo bánh, snack, ca phê pha sẵn, khăn giấy Mỗi cửa hàng được miêu tả qua tên, địa chỉ và một ký hiệu cho biết quy mô của nó (z, zz và zzz tương ứng với cửa hàng loại nhỏ, vừa và lớn) Mỗi cửa hàng được điều hành bởi cửa hàng trưởng, một vài nhân viên tại quầy tính tiền và các nhân viên hậu cần Chính sách giá cả và chế độ khuyến mãi của tất cả các chuỗi bán lẻ thuộc Z này được thực hiện đồng nhất trong

cả nước (ví dụ mì gói trong ly được khuyến mãi nửa giá trong cả tuần, giá bán snack

cả nước (ví dụ mì gói trong ly được khuyến mãi nửa giá trong cả tuần, giá bán snack giống nhau trong toàn quốc) Tuy nhiên mỗi cửa hàng cần quản lý nhập xuất số lượng hàng của riêng mình (ví dụ cà phê đang được khuyến mãi nhưng có thể hết hàng tại một cửa hàng nào đó).

Hệ thống phần mềm này giúp nhân viên quầy thu tiền quét mã vạch và in hóa đơn cho khách Trong hóa đơn có ghi ngày giờ giao dịch, tên nhân viên thu tiền và địa chỉ cửa hàng và tất nhiên danh sách các mặt hàng đã mua cùng với tổng số tiền Phần mềm này cũng giúp trưởng cửa hàng tạo báo cáo thống kê theo ngày, tuần, tháng hay quý cũng như kiểm tra các mặt hàng sắp bán hết Phần mềm có chức năng xác thực bằng thẻ (đối với nhân viên tại quầy tính tiền) cũng như thông qua mật khẩu (trưởng cửa hàng).

Z chain - Object/Class Identification

• Could they be all objects/classes?

• Some of them really are objects Others could simply be the attributes of them.

attributes of them.

• Populate your Classes or Objects

Z chain - Interface Design

Open source development

development in which the source code of a software

system is published and volunteers are invited to

participate in the development process

(www.fsf.org), which advocates that source code should not be proprietary but rather should always be available for users to examine and modify as they wish

Internet to recruit a much larger population of volunteer developers Many of them are also users of the code

Open source systems

Linux operating system which is widely used as a server system and, increasingly, as a desktop environment

Apache web server and the mySQL database

Apache web server and the mySQL database

management system

Open source issues

open source components?

software’s development?

Open source business

source approach to development

product but on selling support for that product

will allow software to be developed more cheaply, more quickly and will create a community of users for the

software

Open source licensing

that source code should be freely available, this does not mean that anyone can do as they wish with that code

• Legally, the developer of the code (either a company or an

individual) still owns the code They can place restrictions on how it

is used by including legally binding conditions in an open source software license

• Some open source developers believe that if an open source

component is used to develop a new system, then that system

should also be open source

• Others are willing to allow their code to be used without this

restriction The developed systems may be proprietary and sold as closed source systems.